Ductile iron pipe is commonly used for water distribution including potable water and sewage transport. The predominate raw material of pipe substrate is ductile iron, a spheroidized graphite cast iron. Like most ferrous or ferrite materials, ductile iron is susceptible to corrosion including galvanic, electrolytic corrosion, and/or microbiologically influenced corrosion. Thus, in the context of a ductile pipe application, it is advantageous to coat or line either the inner surface or outer surface or both of the pipe to mitigate corrosion concerns and extend life expectancy of the iron pipe insitu.
Prior art attempts at lining and/or coating the pipe include polyethylene sleeving by wrapping the pipe in a loose sleeve of polyethylene. Sleeving inhibits the corrosion by physically separating the pipe from soil particles, and preventing direct galvanic corrosion, and provides a barrier that is impermeable to ground water and inhibits the diffusion of oxygen to the ductile iron surface, thereby limiting the availability of electrolytes that would accelerate corrosion. However, use of polyethylene sleeves or encasements has several drawbacks.
Encasings can detach from iron pipes or develop holes or tears. Use of polyethylene sleeving is limited to certain soils or conditions including low moisture conditions, low soil contaminates and low soil resistivity. In addition, polyethylene is vulnerable to degradation by ultraviolet light, so storage, transport and transfer of the pipes requires cost intensive covered trucks and storage facilities.
Other attempts have been made to line the pipes using adherent coating, such as zinc, tar epoxy, or asphalt emulsion coatings, bituminous or polymer finishing layer. A disadvantage of any of these coatings is the lengthy time to dry that the coatings require during the manufacturing process. Because these coating(s) generally take up to thirty to forty five minutes to dry, the coating often rubs off on the production lines and can wash off if exposed to high winds or precipitation. In addition, partially dry coatings often stick or adhere to other pipes in production or other substrates, causing the pulling off of the coating of the pipe's surface (“pull-off” or “blocking”). Consequently, the production lines must significantly slow down line speeds, because compromised coatings result in unacceptable pipe production, and production must be slowed to a pace to accommodate those disadvantages.
Thus, there exists a need in the art for a coating which can be applied to a substrate such as a ductile metal substrate, such as that used in iron pipe, which has an increased rate of drying, blocking or pull-off resistance, and is able to avoid being washed off when exposed to precipitation.